In order to ensure stability while sailing, a sailboat requires a system to counterbalance the thrust of the wind in the sails. For convenience, we shall denote a first longitudinal axis of a sailboat, running from bow to stern, as the X-axis. A second, transverse axis, running sideways from port to starboard will be called the Y-axis.
Overview of the Theory
As illustrated in FIG. 1a, the centre of buoyancy B is the centre of gravity of the volume of water 003 that is displaced by the hull 001. When a hull is subjected to no wind force, the hull's centre of gravity G and the centre of buoyancy B are on a same line, which is substantially perpendicular to the body of water 002.
As illustrated in FIG. 1b, a sailboat begins to tilt, or heel in the Y-axis, as soon as the wind presses into the sails 004. In all types of sailboats, either monohull or multihull, stability of the hull is in that case achieved by taking support on the water on one side (leeward) and using their weight on the other (windward).
When a vessel is heeled, the centre of buoyancy of the ship moves laterally, as shown in FIG. 1b. The righting arm GZ (or righting level) is the horizontal distance between the projection of the centre of buoyancy B and the projection of the centre of gravity G.
When the distance GZ between the fulcrum on water B and the centre of gravity G increases, the Righting Moment RM increases. The Righting Moment is the torque expressing the tendency of the hull to swing back into the position perpendicular to the waterline 002. Therefore, if the RM has a higher value, the sails of the boat can be larger as a higher wind force can be compensated. It follows that a higher RM indicates a higher speed potential of a hull. In boating, the power of a sailboat (provided by its sails area) is a compromise between two factors: the available RM and the “admissible discomfort” produced by the heeling. A sailboat designed to have a good RM is a boat having stiffness.
The sails' thrust force is situated at several meters above the sea level, while the hydrodynamic resistance force is located some decimeters under water. Both forces generate a tilting moment. The sails' thrust force becomes displaced by overboard when the sailboat is heeled.
The total aerodynamic force Fs is the sum of all forces generated by the sails and the rig. Fs is composed of the forward thrust Fx which is equal and opposite to the water resistance Rx (the water drag of the hull and appendages in the X-axis); the drift force Fy which is equal and opposite to the anti-leeway force Ry; the vertical force Fz, turned downward, which is to be added to the mass of ship.
As illustrated in FIG. 2, a torque is applied to the ship through the action of the force Fx. The arm of that torque is the height h. This torque affects the ship attitude relative to its horizontal X-axis and is similar as the heeling torque acting in the Y-axis. When the torque Fx.h inclines the vessel forward, the ship is pushed down by the head: it is trimmed by the head. The angle of trim τ is the incline of the vessel measured in the X-axis.
Wave making resistance is a form of drag that affects surface watercrafts, such as boats and ships, and reflects the energy required to push the water out of the way of the hull. This energy goes into creating the wake.
Discussion on the Y-Axis Aspect
Traditionally, lateral stability of a hull is provided by various means, depending on the type of ship under consideration. Multihull vessels, such as catamarans, have several hulls, thereby increasing the distance GZ while heeling. Big centreboard boats have inside or outside ballasts to lower their centre of gravity, keelboats have an outside fixed ballast also called fin keel. Some race keelboats have one or two canting keels, which allow the displacement of the boat's centre of gravity in order to increase the righting moment. This allows the hull to sail almost flat, thereby increasing its speed capabilities. Above all, canting keels are not well suited for boating in a port or at anchor. They are fragile elements that increase the draft of the boat, which is the distance between the waterline and the deepest point of the boat's structure, and may therefore be a recurring source of damage when navigating in shallow waters.
In addition, all these sailboats may have several inside ballast tanks, which are increasing their weight.
Discussion on the X-Axis Aspect
On the effect of the X-axis torque, the angle of trim increases when the thrust of the sails is increasing. Moreover, the Fz force is situated ahead of the centre of gravity—on the fore part of the ship—and exists at all courses once the ship is heeling. The X-axis torque and Fz force give rise to a loading effect on the fore section, which pushes the ship's nose down into the bow wave.
This loading effect involves the planes formed by fore walls of the hull, which act as an anti-drift. It significantly modifies the wetted areas of the hull. This has a consequence that the position of the anti-leeway force (whose centre is Ry) is moved forward. Furthermore the Fx point is swaying with the vessel movements. The points Rx and Ry are not static either, as they move back and forth. This leads to a situation of instability, in particular when the Fx point gets into a position that is situated behind the point of hydrodynamic resistance and the boat is subject to swings. That usually results in an involuntary course change in the best case, in a boat lying on the water, or in the worst case in a broken mast. Downwind sailing, in strong wind conditions, it will make sailboats too weather helm and unsteady on their way, which compromises seriously the safety of boats and crews.
Very few sailboats are equipped with a system for restoring or adjusting the trim angle. The simplest form of correction of the trim angle, used in small and medium sailboats, is so-called “live ballast”, i.e. the weight of the crew. But this necessitates the presence of a crew and forces the crew to remain in a determined place.
Pleasure sailboats cannot exploit the trim tabs, which are used in motorboats, since it is necessary that the ship has a certain velocity so that the trajectory change of the water has a lifting effect on the ship attitude. This speed condition is certainly not achieved in sailing by pleasure boats.
In sailing races, many competitors use ballast tanks. This is an elegant solution insofar as water is abundant outside of the boat. When sailboats are equipped with ballast tanks, these are used not only to correct the trim angle, but also to increase the stiffness, by increasing the weight of the boat and hence its righting moment. However, water ballasts have imperfections: filling and draining problems due to factors like their position, clogging and ventilation of the strainers, inherent slowness of the system, overload when using, volumes occupied by the tanks on each sides in the accommodations, etc. These drawbacks make water ballast unsuited for boating.
Discussion about the Wave Making Aspect and Speed
Bows are designed to have a cutting effect in waves. This is achieved by providing a stem ending near the waterline by a forefoot (the part of a ship at which the prow joins the keel) and forming two walls. That kind of shape allows flattening the bottom of the hull, which is desirable to reach speed, and naturally makes the sidewalls of the hull more curved than the bottom, especially around the beam.
Therefore when sailing heeled, the hull waterlines are more curved. A consequence of this is an increased wave making resistance.
Moreover, the more the sails are tilted, the more significant is the Fz force, and the further the described loading effect pushes the boat's nose into the bow wave. These phenomena together worsen the depth of the bow wave and increase the resulting braking force. This in turn further increases the wave formation by the hull and therefore impacts negatively on the boat's speed performance.
There are therefore several disadvantages to having ballast fixed down the fin or centreboard. Principally, it is required to have several degrees of heeling before the righting moment becomes significant. Another drawback of such arrangements is that sailboats with fixed ballast remain unable to reach high speed by sailing heeled.
The more the boat is designed to go fast, the more it requires stiffness and trim correction. As a result, movable ballast systems have been proposed in the prior art.
EP-1-1 110 857 discloses a movable ballast system for a ship, the ballast being supported by lateral rails. The disclosed device does not allow balancing a longitudinal charge of the boat
AU 2006 201 460 B1 discloses an adjustable ballast arrangement for a watercraft. As can be seen in the Figures example, the arrangement extends transversely thoroughly outside of the hull. Such a configuration is not capable of solving the balance of the longitudinal charge produced by the sails when the boat is sailing.
WO 91/19641A discloses an arrangement that is able to displace the balancing weight in a sailing boat, using a transversal rail. The mast must swing athwart ship to actuate the ballast. This concept appears to deteriorate the thrust force of the sails by acting more overboard, which generates an even more significant loading vector on the fore part of the boat. The suggested solution does not provide for the balancing of the longitudinal charge produced by the sails when the boat is sailing; at the contrary, it appears to amplify the problem.
Document U.S. Pat. No. 4,867,089 discloses various arrangements for moving an outside ballast element. Such a system however worsens the effects of water drag due to more immersed parts.
Document WO 92-16409 discloses a system intended to be a complement of water ballasts in ships. The ballast elements can only be moved along fixed trackways which define several crossings along two axes. The crossing points have to be used to change movement directions. The system is therefore not suitable for changing the ballast position quickly and precisely, which is required for efficient operation on a sailboat.
Document WO 01/47769A discloses a movable ballast arrangement for a boat. The arrangement is located in a conduit, which itself is preferably located inside the hull. The arrangement involves a closed loop tunnel, which contains spheres of different sizes. These ballast spheres are moved inside the tunnel by means of a worm gear, which engages through an opening in the tunnel with the smallest spheres. Considering the length of the described loops, the transfer of ballast from one side to the other takes about 20 seconds. However, a tack in real life is made in about 5 to 7 seconds; hence the proposed system would not appear to react quickly enough. More importantly, it is not possible to adjust both the trim angle and the heeling angle precisely and independently.
Another known ballast system has been disclosed by the applicant in WO/2009/026964. The system provides a mobile ballast, moving in a watertight tunnel in a horseshoe form. In order to balance any longitudinal charge, the ballast has to be moved laterally first. Likewise, this system is inappropriate for balancing a sailboat when it is sailing by wind stern, i.e., without heeling in the Y axis.